The invention relates to a sorter, and particularly a screen sorter, for paper pulp suspensions. The sorter is of the kind comprising a stationary, rotationally symmetrical wire cage, in the peripheral vicinity of which screening vanes are pivoted, an inlet for feeding the suspension into the cage, a pulp chamber surrounding the cage and a pulp outlet from the pulp chamber.
Such a sorter is known from U.S. Pat. No. 3,692,183.
Such sorters, in which the wire cage has appropriately fine perforations, are also used a short distance in front of the breast box or headbox of a paper machine for fine grading. It appears that the screening vanes rotating in front of the wire cage produce ripples in the flow of suspension which become noticeable as far back as the breast box and which give rise to streakiness in the paper because of fluctuations in the surface-related pulp. One remedy is to use the subdivided screening vanes as described in the above noted U.S. patent.
Moreover, a damper consisting of a tall, upright cylinder having an upper cushion of air has been added to the pipe leading to the breast box (see "Das Papier", Book 1, 1980, Page 8). In accordance with German Patent Specification No. 27 51 949, for this purpose, another special device has been developed in which the suspension passes through a cylindrical annulus, which is separated from a damping chamber by an elastic membrane. In the damping chamber is fluid, on the upper, free surface of which a cushion of air rests, and the pressure of the air cushion can be controlled. However, such a pressure pulsation damper is relatively expensive and requires more space. Furthermore, according to the first named document, it appears that the pulsations coming from a vertical screen cannot be eliminated with such a damping device.